EP-A-372820 discusses such overvoltage protected switches. In particular EP-A-372820 discusses the use of clamping diodes to protect power MOSFETs used, for example, to switch inductive loads. EP-A-372820 describes the previous use of a clamping diode connected externally between the gate and drain (the first main electrode in this case) of a power MOSFET and having an avalanche voltage about two to three volts less than the avalanche voltage of the power MOSFET so that when the drain voltage rises while the power MOSFET is switched off (for example as may occur when switching off an inductive load) and exceeds the avalanche voltage of the clamping diode, the resulting avalanche current causes the gate voltage of the power MOSFET to rise to switch on the power MOSFET and so allow at least part of the energy to be dissipated by conduction of the power MOSFET, thereby avoiding potentially destructive avalanche conditions within the power MOSFET. As an alternative to an externally connected clamping diode, EP-A-372820 proposes the use of a chain of polycrystalline silicon diodes to form the clamping diode. This has the advantage that the polycrystalline silicon diodes can be provided on top of the power MOSFET so that the use of an externally connected diode is avoided. In addition, by providing back-to-back polycrystalline silicon diodes a temperature compensation effect can be achieved. However, the properties of polycrystalline silicon diodes can be very variable and their current-handling capabilities are limited.
It is an aim of the present invention to provide an overvoltage protected switch in which the protective means can be integrated with the power semiconductor device to be protected.